OAuth: What am I receiving invalid_client mean when trying having access token ?

Hello,

I am implementing OAuth and I am having an error that I don’t understand

Here is a record of the video showing the test 

Here is the code use to send the request for getting access token

export const getFrontAccessToken = async (code: string, slug: string) => {
  const clientId = process.env.NEXT_PUBLIC_FRONT_CLIENT_ID;
  const clientSecret = process.env.NEXT_PUBLIC_FRONT_CLIENT_SECRET;
  const encodedAuth = btoa(`${clientId}:${clientSecret}`);
  console.log('getFrontAT ', clientId, clientSecret, encodedAuth);

  const response = await fetch('https://app.frontapp.com/oauth/token', {
    method: 'POST',
    body: JSON.stringify({
      code,
      // redirect_uri: `http://localhost:3000/settings/${slug}/front`,
      grant_type: 'authorization_code',
    }),
    headers: {
      'Content-Type': 'application/json',
      accept: 'application/json',
      Authorization: `Basic ${encodedAuth}`,
    },
  });
  console.log(await response.json());
};